As methods for electrically braking a driven motor, there are a reverse brake control which electrically brakes by applying current to the coil of the motor in the direction opposite to the direction at the time of the normal rotation of the motor, and a short brake control which electrically brakes the motor by a current caused by a counter electromotive voltage that is generated by the rotor magnetic field and the stator coil (by the stator magnetic field and the rotor coil in the case of an inner rotor type motor) at the time when both ends of the motor coil are short-circuited.
FIG. 11 shows changes in rotation speed of a motor at the time when the motor is electrically braked by a conventional control system. The broken line S11 shows a case where only the reverse brake control is performed, and the broken dotted line S12 shows a case where only the short brake control is performed. Further, the two-dot chain line S13 shows a case where the control is performed by the control method disclosed in Japanese Patent Laid-Open No. 2007-68400.
As shown by the broken line S11 in FIG. 11, when the motor which is driven for steady rotation is electrically braked only by the reverse brake from the time T0, the rotation speed is rapidly reduced because the braking force of the reverse brake is large. However, in the reverse brake control, current is applied to the motor coil in the direction opposite to the direction at the time of the normal rotation. Thus, when the current continues to be applied in the reverse direction after the rotor is stopped, and when the current is applied in the reverse direction until the rotor is rotated at low speed immediately before being stopped, the rotor is rotated in the reverse direction. Therefore, in order to prevent the reverse rotation, it is necessary to provide a reverse rotation detection circuit, or to stop the reverse-direction current application at a stage earlier to some extent than the time when the rotor is stopped.
In the case where the rotor is stopped by providing the reverse rotation detection circuit, a time up to the time T3 is required until the rotor is stopped. Further, in the case where the reverse-direction current application is stopped at a stage earlier than when the rotor is continuously, the rotor continues to be rotated by its own inertia, and hence a time longer than the time up to the time T3 is required until the rotor is stopped.
On the other hand, in the case where a motor which is driven for steady rotation is electrically braked only by the short brake, the reverse rotation is not caused as shown by the broken dotted line S12 in FIG. 11. However, since the braking force depends on the back electromotive force, the braking force is also reduced as the rotation speed is reduced, so that a time up to the time T4 is required until the rotor is stopped.
Here, as a conventional control circuit which stops a motor in a short time without requiring the reverse detection circuit, there is described in Japanese Patent Laid-Open No. 2007-68400 a system which performs switching between the reverse brake control and the short brake control according to the rotation speed of the rotor.
In the control system described in Japanese Patent Laid-Open No. 2007-68400, as shown by the two-dot chain line S13 in FIG. 11, the switching between the reverse brake control and the short brake control is performed in dependence upon the rotation speed of the rotor, so as to reduce the rotation speed of the rotor in a short time up to a number of rotation N1 set as a threshold value immediately before the rotor is stopped, and then the short brake control is performed from the time T1 when the rotation speed is reduced lower than the rotation speed N1 set as the threshold value immediately before the rotor is stopped. When such control is performed, it is possible to stop the rotor at the time T2 of shorter time than the time when the rotor is stopped only by one of the reverse brake control and the short brake control.